1. Field of the Invention
The present invention relates a scanning optical device used in image formation of an electrophotographic type and an image forming apparatus having the same.
2. Description of the Related Art
In an image forming apparatus of an electrophotographic type, scanning is performed by reflecting a beam from a light source by a rotating rotary polygon mirror, and the beam is irradiated on a photosensitive drum serving as an image bearing member to form an electrostatic latent image. In recent years, even in an image forming apparatus of an electrophotographic type, a color image is strongly demanded. For this reason, a plurality of photosensitive drums (in general, four photosensitive drums of yellow, magenta, cyan, and black) are arranged. Optical scanning is performed to the photosensitive drums to form toner images of different colors. The toner images are superposed on each other to obtain a color image.
In the color image forming apparatus as described above, as a scanning optical device which performs optical scanning to the plurality of photosensitive drums, as shown in FIG. 7, a scanning optical device in which a plurality of laser beams are scanned by one rotary polygon mirror may be used (since the scanning optical device in FIG. 7 is horizontally symmetrical, reference numerals in FIG. 7 on only one side are shown).
The scanning optical device shown in FIG. 7 uses a scheme in which two laser beams are incident on both the sides of a polygon mirror 28 serving as one rotary polygon mirror to expose photosensitive drums by irradiated beams E1 to E4. The optical arrangement is an oblique incident optical system and has a configuration in which a second image forming lens is arranged after laser beams are separated from each other.
In this case, in the oblique incident optical system, as shown in FIG. 8, when a plane (in FIG. 8, an X-Y plane) defined by a normal line of a reflecting plane of the polygon mirror 28 and a rotating direction of the polygon mirror 28, as shown in FIG. 9, a laser beam is incident at a predetermined angle with respect to the reference plane (The incident angle will be called an “oblique incident angle” hereinafter.). In this manner, upper and lower optical paths are separated from each other behind an outgoing laser beam from the polygon mirror 28.
The scanning optical device is shielded from the outside by a dust-tight glass 32 to protect the scanning optical device from dust, an optical box 33 in which optical elements are built in, and an upper lid 34 to which the dust-tight glass 32 is fixed and which seals the optical box.
Two laser beams emitted from the polygon mirror 28 transmits through a first imaging lens 29, and the laser beams transmitting through a photosensitive drum is reflected downward by a separation folding mirror 31c. Since laser beams are incident on the first imaging lens 29 at angles different from each other, the first imaging lens 29 is constituted by a cylinder lens. An image is formed in a sub-scanning direction by second imaging lenses 30 which are arranged for respective optical paths of the laser beams.
A laser beam E2 crosses the other laser beam and goes downward. The beam transmitted through the second imaging lens 30 arranged on the way, is reflected again by a folding mirror 31b arranged on the lower surface of the optical box, and is irradiated on the photosensitive drum through a side of the first imaging lens 29. In this case, laser beams E1 and E4 irradiated on the photosensitive drums on both end portions transmit immediately under the separation folding mirror 31c, transmits through the second imaging lens 30, and then are irradiated on the photosensitive drums by a folding mirror 31a. The separation folding mirror 31c is arranged such that vignetting of the beams of two laser beams is prevented from occurring by tolerances of parts, optical facet angle error of a polygonal motor, and the like.
The scanning optical device which employs the oblique incident optical system is an optical system which can perform deflection and scanning of a plurality of beams at once while keeping a unit compact.
However, on the other hand, in comparison with a optical system in which an oblique incident angle is 0, i.e., a beam is incident perpendicularly to a reflecting surface of the polygon mirror 28, the oblique incident optical system theoretically deteriorates in pitch unevenness (to be referred to as an “optical facet angle error” hereinafter) caused by an optical facet angle error. This is because the reflecting plane is eccentric with respect to a rotating shaft of the polygonal motor.
FIG. 9 shows a beam track near the polygon mirror in the oblique incident optical system. FIG. 9 shows a state in which a beam is incident on the polygon mirror 28 which is eccentric by d with respect to the rotating shaft at an oblique incident angle α. In general, this eccentricity is caused by two factors, i.e., a fluctuation of the polygon mirror itself and a play occurring between the rotating shaft of the motor and the polygon mirror 28 (the eccentricity with respect to the rotating shaft of the reflecting plane of the polygon mirror will be called “plane eccentricity” hereinafter).
As shown in FIG. 9, when plane eccentricity of d occurs in the polygon mirror 28 with respect to the rotating shaft, the reflecting plane shifts by d while the polygon mirror 28 rotates once. In the oblique incident optical system, a reflecting position shifts on the polygon mirror 28 by the plane eccentricity, and the beam shifts in a sub-scanning direction as indicated by a broken line. As a result, a sub-scanning shift having a frequency (rotational frequency of the polygon mirror 28) which is equal to that of an optical facet angle error) occurs. Since an optical facet angle error component caused by the plane eccentricity is deteriorated in proportion to an incident angle, the optical facet angle error component must be suppressed to a low level as much as possible.
In a conventional technique, since the oblique incident optical system is used, in order to suppress the eccentric component, a plurality of projecting portions may be formed around a rotating shaft to which the polygon mirror is attached, and the polygon mirror may be fixed by being caulked by the projecting portions. In this manner, the polygon mirror is fixed by caulking, fastening screws and holes for the screws are not necessary, and a fluctuation of weight with respect to a rotational center of the connected polygon mirror is reduced, so that eccentricity is reduced (Japanese Patent Application Laid-open No. 9-21974).
As another example, a play is set between the polygon mirror and the rotating shaft, and an amount of plane eccentricity and an amount of optical facet angle error are adjusted. Thereafter, the polygon mirror and the rotating shaft are fixed with an ultraviolet adhesive agent, so that pitch unevenness or the like is decreased (Japanese Patent Application Laid-open No. 2004-102006).
Parts such as the rotating shafts of the polygon mirror and the motor related to an amount of eccentricity are increased in precision and improved in adjusting precision to make it possible to further reduce the optical facet angle error.
On the other hand, in some scanning optical device of an oblique incident optical system, as shown in FIG. 7, deflection and scanning are not symmetrically performed to the polygon mirror 28, a polygon mirror is arranged at an end portion of the scanning optical apparatus to perform deflection and scanning of all beams on the same plane.
In particular, in Japanese Patent Application Laid-open No. 2004-287237, the following configuration is described. That is, deflection and scanning of a plurality of beams are performed on the same plane of the polygon by oblique incidence, and an incident angle of a laser to form yellow and black toner images is increased.
In contrast to this, in such a scanning optical device, oblique incident angles must be different from each other to separate beams from each other. For this reason, an optical path having an oblique incident angle larger than those of other optical paths is consequently generated.
When the oblique incident angle becomes large, even though an optical facet angle error is small, a shift of a beam caused by the optical facet angle error is larger than that obtained when the oblique incident angle is small. For this reason, when a beam having a large oblique incident angle exposes an image bearing member for forming a toner image constituted by a toner having a low brightness, a color shift is conspicuous because the toner image having the low brightness is easily conspicuous.